Lymphocytes of inbred mouse strains have been tested for the presence of gp70, the major glycoprotein of murine leukemia viruses, on the cell surface. Some strain (NZB, DBA/2, NZW) were shown to express high levels of cell surface antigen whereas other (C57BL/6, NFS, SWR) expressed low levels. In crosses between DBA/2 and C57BL/6 mice, a semidominant gene was found to regulate the level of antigen expression. The gene is linked to Fv-1 on chromosome 4. Other studies demonstrated that mouse thymic virus which is tropic for T cells modifies the functional activity of T cells participating in the graft-versus-host reaction and those T cells which modify the B cell response to Type III pneumococcal polysaccharide. BIBLIOGRAPHIC REFERENCES: Cross, S.S., Morse, H.C.III, and Asofsky, R.: Neonatal infection with mouse thymic virus: Differential effects on T cells mediating the graft-versus-host reaction. J. Immunol. 117: 635-638, 1976. Morse, H.C.III, Cross, S.S. and Baker, P.J.: Neonatal infection with mouse thymic virus: Effects on cells regulating the antibody response to Type III pneumococcal polysaccharide. J. Immunol. 116: 1613-1617, 1976.